1. Field of the Invention
The present invention relates to a light pipe which can be used in combination with a point light source to form a planar light source unit excellent in luminance and, which can be used to form a reflection type liquid-crystal display device free from moire, bright and easy to view.
The present application is based on Japanese Patent Application No. 2000-323467, which is incorporated herein by reference.
2. Description of the Related Art
There has been proposed a front-light reflection-illumination type liquid-crystal display device in which a planar light source unit constituted by a side light pipe is disposed on the visual side of a reflection type liquid-crystal panel so that display can be viewed in a dark place or the like. Such a front light system can use a liquid-crystal panel having a reflection layer with a high reflectance. Hence, the front light system has an advantage, in enhancement of luminance in a reflection mode, over a backlight system which requires a semi-transmission type of a half-silvered mirror for forming a reflection-illumination type liquid-crystal display device.
A plate-like body 8 having a stripe-like light exit means B provided in its upper surface as shown in FIG. 5 was heretofore known as a light pipe allowed to be used in the aforementioned front light system. A light source was disposed on an incidence side surface 8c of the light pipe 8 to thereby form a planar light source. In this case, a linear light source such as a fluorescent lamp was disposed because the angle of incidence of light onto the light exit means B varied largely in accordance with the place where the point light source is arranged so that light is emitted only partially from the light pipe. In the front light system, however, display light was viewed through the light pipe. There was a problem that moire was apt to occur because the stripes of the light exit means B interfered with pixels disposed tessellatedly or zigzag in the liquid-crystal panel.
On the other hand, a light pipe having a light exit means constituted by a structure of spherical dots or sectionally cylindrical pits to thereby perform light emission by a point light source was also known. In the light pipe, however, light was reflected by a spherical or cylindrical surface. Hence, light was diffused largely so that exit light was inclined largely with respect to the normal line direction (frontal direction) of the light pipe. There was therefore a problem that the percentage of light entering the reflection type liquid-crystal panel as illumination light was too small to obtain bright display.
An object of the present invention is to develop a light pipe which can be used in combination with a point light source to form a compact planar light source unit excellent in uniform light emission from the whole surface thereof and small in light source-disposing space and, which can be used to form a reflection type liquid-crystal display device excellent in brightness, free from moire and excellent in display quality both in a reflection mode and in an illumination mode.
According to the present invention, there is provided a light pipe constituted by: a plate-like body having upper and lower surfaces; and an incidence side surface constituted by a side surface between the upper and lower surfaces, the plate-like body including light exit means formed in the upper surface so that light incident on the incidence side surface is made to exit from the lower surface through the light exit means, the light exit means being in an arrangement to face at least one virtual center on or outside the incidence side surface, whereby incident light from the upper or lower surface is transmitted to exit from the lower or upper surface through the plate-like body. There is further provided a planar light source unit constituted by the light pipe, wherein a point light source is disposed in the virtual center of the light exit means of the light pipe or between the virtual center and the incidence side surface; and a reflection type liquid-crystal display device constituted by the planar light source unit, and a liquid-crystal panel including a reflection layer disposed on a lower surface side of the light pipe in the planar light source unit.
According to the present invention, a point light source is disposed in the virtual center, or the like, for the light exit means of the light pipe so that uniform light emission from the whole lower surface of the light pipe can be obtained. Hence, a compact planar light source unit small in light source-disposing space can be formed. The planar light source unit can be used to form a reflection type liquid-crystal display device excellent in brightness, free from moire and excellent in display quality both in a reflection mode and in an illumination mode.
Particularly when a light exit means constituted by pit-like minute concave or convex portions each having an optical path changing surface inclined at an angle in a range of from 35 to 48 degrees with respect to a reference plane of the lower surface is provided as means by which light incident on an incidence side surface is reflected toward the lower surface, light transmitted in the light pipe radially from the point light source can be made to exit with good directivity from the lower surface through the optical path changing surfaces in the normal line direction. Moreover, the density of the arranged optical path changing surfaces can be adjusted easily. Hence, the influence of the light pipe on light transmitted vertically through the light pipe can be controlled through the area occupied by the light exit means. Hence, the influence of the light pipe on liquid-crystal display can be suppressed to attain improvement of display quality.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.